Detecting a white line drawn on a road surface is disclosed, for example, in Japanese Laid Open Patent Application No. 15,605/1989 where a scene including a road is surface which is located ahead of a road vehicle and which is obtained by a photographing unit mounted onboard the vehicle is displayed on a screen based on X, Y orthogonal coordinates. Utilizing image data displayed on the screen, derivatives in a direction which forms an angle of 45.degree. with respect to the X-axis are calculated, and those derivatives which have magnitudes in excess of a given value are chosen as feature points represented in X, Y orthogonal coordinates. A succession of such feature points are processed into a thin line, and a thin line having a length greater than a given value is picked out to be recognized as representing a white line drawn on the road surface when such thin line satisfies conditions which are preset on the basis of the features of such white line appearing on the screen. However, when the road surface is relatively dark or when the white line is vanishing, the road surface condition becomes different from a road surface condition which is used as a prerequisite by the detecting device. As a result, a high probability of false recognition or inability of detection exists.
Recently, the use of "Hough transformation" in detecting lines on a screen has been proposed. (See, for example, an article appearing on pages 1769-1776 of Journal of the Association of Electronics and Communication, October 1985, Vol. J68-D No. 10). When each point of a rectilinear line on a screen represented in X-Y orthogonal coordinates is converted into .rho.,.theta. polar coordinates, each point will be converted into a curve in the polar coordinates. Thus, curves representing individual points on a single rectilinear line in and the X-Y orthogonal coordinates will intersect at a point, and the coordinates (.rho., .theta.) may be substituted into a conversion formula for converting the coordinates (.rho., .theta.) into the X-Y orthogonal coordinates to derive an equation representing the rectilinear line in the X-Y orthogonal coordinates. In the detection of a rectilinear line utilizing the Hough transformation, a rectilinear line representing a set (distribution) of black points, the edges of an image, or the like which are dispersed throughout the screen (hereafter referred to as feature points) may be detected. Specifically, each feature point (Xs, Ys) in the X-Y orthogonal coordinates may be sequentially converted into a plurality of coordinates (.rho., .theta.) through which curves in the .rho., .theta. polar coordinates, and corresponding to the respective feature points, pass. Frequency data which is allotted to each one of polar coordinates (.rho., .theta.) in memory may be incremented. Such operation is executed for all the feature points. A particular polar coordinate (.rho.m, .theta.m) representing a maximum value of the frequency data among the memory is then picked out, and the rectilinear line represented by the particular polar coordinate (.rho.m, .theta.m) is determined to be the line which approximates the succession of the feature points.
More specifically, a single point in the X-Y orthogonal coordinate system becomes a single curve in the .rho., .theta. polar coordinate system. Accordingly, a single point in the X-Y coordinates is converted into a plurality of sets of coordinates through which a single curve in the .rho., .theta. polar coordinates passes. The .rho., .theta. polar coordinates are used as an address for memory means, and the data stored at an address corresponding to the respective coordinates is updated by incrementing the data by one. If a similar operation is executed for other points lying on the same rectilinear line in the X-Y orthogonal coordinates, which are assumed to be a total of E points, data representing the number "n" will be n stored at a single address (or a single point .rho.m, .theta.m in the .rho.-.theta. polar coordinates). Such data "n" indicates that n-curves will intersect with each other at this address. Similarly, data at other addresses will represent either 0 (no curve passing therethrough) or 1 (a single curve passing therethrough). Thus it will be seen that the address (.rho.m, .theta.m) in which data representing "n" is stored defines the rectilinear line in the X-Y orthogonal coordinates. This line is defined as follows: EQU .rho.m=X cos .theta.m+Y sin .theta.m
When the described operation is performed for a plurality (n) of points, which may be a group of feature points, for example, in the X-Y orthogonal coordinates and for which it is uncertain whether they lie on a common rectilinear line, data exhibiting a value less than n will be distributed in the memory means if a point or points which do not lie on the same rectilinear line are contained in the group. When the address (.rho.m, .theta.m) of data representing a maximum value is picked out from such data, it then follows that a rectilinear line represented by the equation EQU .rho.m=X cos .theta.m+Y sin .theta.m
in the X-Y orthogonal coordinates is a line which is representative of a bank of rectilinear lines joining any pair of points in the group.
With the detection of a rectilinear line using the Hough transformation, it follows that in the event the edge of an image is roughened, there will be obtained a single rectilinear line (.rho.m, .theta.m) which statistically represents a number of imaginary lines joining any pair of points. Thus, one point located on the edge of the image and any one of a plurality of dispersed feature points can be used to yield a high reliability in detecting a rectilinear line. This may be utilized in detecting a white line appearing in an image which is obtained by taking a picture of a scene located ahead of a vehicle with an onboard television camera, with a concomitant reliability.
However, since each feature point (Xs, Ys) in the X-Y orthogonal coordinate system becomes a curve in the .rho.-.theta. polar coordinate system, a conversion of one of the feature points (Xs, Ys) into the .rho.-.theta. polar coordinates requires an excessive repetition. For example, assuming that the extent of .rho. is limited by 0.ltoreq..theta.&lt;90.degree., and .theta. is scanned in a unit of 1.degree. each value of .theta. from 0.degree. to 89.degree. must be input to the equation EQU .rho.=Xs cos .multidot..theta.+Ys sin .theta.
to derive a .rho. value corresponding to each value of .theta.. This means that an increased calculation time as well as an increased capacity of memory are required for the conversion, requiring a memory having a capacity of 90.times.90.times.Da (Da represents a number bits used to represent frequency data) for storing such frequency data.
To cope with this problem, an article in the Technical Research Report of the Association of Electronics and Communication, IE86-67, pages 49-56, entitled "On fast Hough transformation utilizing pyramidal hierarchy" has been proposed to reduce the number of feature points which are processed in the calculation by sampling feature points in the original image (which is displayed on the X-Y orthogonal coordinate system). However, the sampling reduces information (feature points) which is used to define the rectilinear line to be detected, it devalues the significance of the statistical processing. In other words, sampling reduces the very merit of the Hough transformation and degrades the reliability. In addition, this technique has no contribution to reducing the number of times the calculation is repeated for one feature point.